Originally Posted by TheSultanofScud

Is there a published letter of interpretation for this, something from a specific FSDO, or something specifically documented elsewhere that supports this?

I was always under the impression that an instructor pilot in such a situation either had to be an ATP—in a 121 or 135 environment—or had to possess the type rating + a flight instructor certificate with cat-class privileges to do the same training in a part 91 operation.

I'm not saying you're wrong by any means, I'd just like to have something published to point to if what you have said is the case.